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Gill rakers

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Gill rakers (above gill arch) from an American Shad Alosa sapidissima are not to be confused with gill filaments (below); they are particularly important for filter-feeding fish (photo credit: NOAA).

While gill rakers have no role in gas exchange, the predominant function of gills,  they do perform an equally important function for filter-feeding fish – food acquisition.  These bony projections serve as a sieve to trap food particles.  They vary greatly in length and number and these characteristic differences have evolved with different feeding strategies for fish.  A plankton feeder, for example, such as American Shad Alosa sapidissima, has very tightly-packed, comb-like, gill rakers to efficiently filter their food from the water column. An omnivore or piscivore, on the other hand, has shorter, more widely-spaced gill rakers, better for larger prey items.  In fact, the morphology of gill rakers is so diverse that they are often used as a taxonomic tool to identify and classify fish species (e.g., gill raker counts can differentiate species on a dichotomous key).

Gills

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The underside of a Walleye’s gills (photo credit: USFWS).

Someone may tell you that you look “green around the gills” when you look ill but the expression doesn’t translate well to fish gills…

No matter if a fish is sick or not, fish gills are often red – blood red, to be exact.  That is because gills have blood vessels very close to their external surface.  As the primary mode of gas exchange for most fish (but not all!), gills absorb oxygen from water that passes over them and release carbon dioxide usually facilitated through a counter-current exchange of blood and water flowing in opposite directions.  Gill lamellae provide increased surface area for gas exchange, which is particularly important because there is less dissolved oxygen in water than there is in air.  For fish that breathe air, lungs and skin provide alternative methods for oxygen absorption.

Ganoid scales

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Diamond-shaped ganoid scales form an interlocking protective armor for fish.

Diamond-shaped ganoid scales form an interlocking protective armor for fish

Ganoid scales are dimond-shaped scales found in lower order fishes such as the bichirs (Polypteridae), Bowfin (Amia calva), paddlefishes (Polyodontidae), gars (Lepisosteidae), and sturgeons (Acipenseridae).  Unlike ctenoid or cycloid scales, ganoid scales are comprised of bone.  They have a bony basal layer, a layer of dentin (also found in human teeth), and an outer layer of ganoine which is the inorganic bone salt for which these scales are named.  These scales interlock with peg-and-socket joints which make them quite inflexible, compared with ctenoid or cycloid scales, but very protective.

Spotted Gar have ganoid scales

Spotted Gar have ganoid scales